During chemotherapy, many cancers develop multidrug resistance (MDR). One key protein underlying MDR is the MDR1-protein (commonly known as P-glycoprotein (Pgp)), which extrudes multiple drugs from the cancer cells utilizing ATP. The functional characteristics of Pgp have been extensively characterized. However, the mechanism by which anticancer drugs induce the expression of Pgp; the mechanism by which the activity of Pgp is regulated in cancers, are unknown. We have identified a new gene that codes for a (38 kDa RING finger protein, termed MDR1Pi. This protein is highly expressed in drug-resistant cancer cells. Anticancer drugs further increase the expression of this protein. Importantly, we show that MDR1Pi interacts with Pgp, specifically the linker region, which is known to join the NH2- and COOH-halves of Pgp. Interestingly, the linker region is shown to be important for the drug-transport and ATPase activities of Pgp. Thus, these findings suggest that MDR1Pi is an important protein, which regulates the Pgp function through its interactions with the linker region. Hence, our major interest is to provide a thorough understanding of MDR1Pi function in the development of MDR that will contribute to future therapeutic approaches to treat MDR cancers using MDR1Pi as a new molecular target. To this end, we will evaluate the function of MDR1Pi in breast and in prostate cancer cells. We have defined three specific objectives to address the functional role of MDR1Pi in the development of MDR, which are to: 1). determine the effects of MDR1Pi on the ATPase and drug transport functions of Pgp; 2). determine the role of MDR1Pi in the development of MDR; 3). determine the role of MDR1Pi in the MDR1 gene expression. In specific aim 1, we will measure and compare the drug transport and ATPase activities of Pgp and Pgp-MDR1Pi complex, to identify the role of MDR1Pi in the Pgp function. Specific aim 2 is designed to determine if MDR1Pi alone imparts MDR phenotype, by expressing MDR1Pi in drug-sensitive cells; and by silencing the expression of MDR1Pi in drug-resistant cells. We will also compare the expression patterns of MDR1Pi and Pgp to determine the functional relationship between MDR1Pi and Pgp. The specific aim 3 is designed to determine if MDR1Pi regulates the MDR1 gene transcription. We will also determine how the linker region and anticancer drugs modulate the MDR1Pi function in relation to MDR1 gene transcription. Characterization of MDR1Pi, the first known interactor of Pgp, will unravel the mechanisms by which the function and expression of Pgp are regulated. These data will be key to the future designs of re-sensitizing the anticancer drug-refractory cancers. [unreadable] [unreadable] [unreadable]